Optical device having a light transmitting substrate with external light coupling means

ABSTRACT

An optical device, includes a light-transmitting substrate ( 20 ) having an input aperture and first and second major surfaces ( 26, 32 ) parallel to each other and edges, one partially reflecting surface located in the substrate which is non-parallel to the major surfaces of the substrate and an external optical arrangement having an output aperture for coupling light into the substrate by total internal reflection. The optical arrangement for coupling light having an output aperture optically attached to the input aperture of the substrate with the part of the substrate located next to the substrate input aperture, being substantially transparent.

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, andparticularly to devices which include a plurality of reflecting surfacescarried by a common light-transmissive substrate, also referred to as alight-guide element.

The invention can be implemented to advantage in a large number ofimaging applications, such as head-mounted and head-up displays,cellular phones, compact displays, 3-D displays, compact beam expanders,as well as non-imaging applications like flat-panel indicators, compactilluminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inhead-mounted displays where an optical module serves both as an imaginglens and a combiner, in which a two-dimensional display is imaged toinfinity and reflected into the eye of an observer. The display can beobtained directly from either a spatial light modulator (SLM) such as acathode ray tube (CRT), a liquid crystal display (LCD), an organic lightemitting diode (OLED) array, or a scanning source and similar devices,or indirectly, by means of a relay lens or an optical fiber bundle. Thedisplay comprises an array of elements (pixels) imaged to infinity by acollimating lens and transmitted into the eye of the viewer by means ofa reflecting, or partially reflecting surface acting as a combiner fornon-see-through and see-through applications, respectively. Typically, aconventional, free-space optical module is used for these purposes. Asthe desired field-of-view (FOV) of the system increases, however, such aconventional optical module becomes larger, heavier and bulkier, andtherefore, even for a moderate performance device, is impractical. Thisis a major drawback for all kinds of displays, but especially inhead-mounted applications, where the system must, of necessity, be aslight and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and, on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box (EMB) of the optical viewing angles resulting from thesedesigns is usually very small, typically less than 8 mm. Hence, theperformance of the optical system is very sensitive, even to smallmovements of the optical system relative to the eye of the viewer, anddoes not allow sufficient pupil motion for convenient reading of textfrom such displays.

For a more detailed background of the present invention, the teachingsincluded in the publications WO 01/95027, WO 03/081320, WO 2005/024969,WO 2006/013565, WO 2006/085309, WO 2006/085310, WO 2007/054928 andPCT/IL2007/000172 in the name of Applicant, are herein incorporated byreference.

DISCLOSURE OF THE INVENTION

The present invention facilitates the design and fabrication of verycompact light-guide optical elements (LOE) for, amongst otherapplications, head-mounted displays. The invention allows relativelywide FOVs together with relatively large EMB values. The resultingoptical system offers a large, high-quality image, which alsoaccommodates large movements of the eye. The optical system offered bythe present invention is particularly advantageous because it issubstantially more compact than state-of-the-art implementations, andyet it can be readily incorporated even into optical systems havingspecialized configurations.

The invention also enables the construction of improved head-up displays(HUDs). Since the inception of such displays more than three decadesago, there has been significant progress in the field. Indeed, HUDs havebecome popular and they play an important role, not only in most moderncombat aircraft, but also in civilian aircraft, in which HUD systemshave become a key component for low-visibility landing operation.Furthermore, there have recently been numerous proposals and designs forHUDs in automotive applications where they can potentially assist thedriver in driving and navigation tasks. Nevertheless, state-of-the-artHUDs suffer several significant drawbacks. All HUDs of the currentdesigns require a display source that must be offset a significantdistance from the combiner to ensure that the source illuminates theentire combiner surface. As a result, the combiner-projector HUD systemis necessarily bulky and large, and requires a considerable installationspace, which makes it inconvenient for installation and at times evenunsafe to use. The large optical aperture of conventional HUDs alsoposes a significant optical design challenge, rendering the HUDs witheither compromised performance, or leading to high cost whereverhigh-performance is required. The chromatic dispersion of high-qualityholographic HUDs is of particular concern.

An important application of the present invention relates to itsimplementation in a compact HUD, which alleviates the aforementioneddrawbacks. In the HUD design of the current invention, the combiner isilluminated with a compact display source that can be attached to thesubstrate. Hence, the overall system is very compact and can be readilyinstalled in a variety of configurations for a wide range ofapplications. In addition, the chromatic dispersion of the display isnegligible and, as such, can operate with wide spectral sources,including a conventional white-light source. In addition, the presentinvention expands the image so that the active area of the combiner canbe much larger than the area that is actually illuminated by the lightsource.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held application such ascellular phones. In today's wireless internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the device of the end-user.The mobility requirement restricts the physical size of the displays,and the result is a direct-display with a poor image viewing quality.The present invention enables a physically compact display with a largevirtual image. This is a key feature in mobile communications, andespecially for mobile internet access, solving one of the mainlimitations for its practical implementation. Thereby the presentinvention enables the viewing of the digital content of a full formatinternet page within a small, hand-held device, such as a cellularphone.

A broad object of the present invention is therefore is to alleviate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

The invention therefore provides an optical device, comprising alight-transmitting substrate having an input aperture and at least firstand second major surfaces parallel to each other and edges, at least onepartially reflecting surface located in said substrate which isnon-parallel to the major surfaces of said substrate, optical meanshaving an output aperture for coupling light into said substrate bytotal internal reflection, characterized in that said optical means forcoupling light is located outside of said substrate, that the outputaperture is optically attached to the input aperture of said substrateand that the part of the substrate which is located next to thesubstrate input aperture is substantially transparent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments with reference to the following illustrative figures, sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 illustrates a span of optical rays which are coupled into alight-guide optical element, according to the present invention;

FIG. 2 is a diagram illustrating an embodiment for coupling light into alight-guide optical element, according to the present invention;

FIG. 3 is a diagram illustrating an embodiment for coupling light into alight-guide optical element utilizing a coupling prism, according to thepresent invention;

FIG. 4 is a diagram illustrating an embodiment for coupling light into alight-guide optical element utilizing two coupling prisms, according tothe present invention;

FIG. 5 is a diagram illustrating another embodiment for coupling lightinto a light-guide optical element utilizing two coupling prisms,according to the present invention;

FIG. 6 is a diagram illustrating a yet another embodiment for couplinglight into a light-guide optical element utilizing two coupling prisms,according to the present invention;

FIG. 7 is a diagram illustrating a yet another embodiment for couplinglight into a light-guide optical element utilizing two coupling prisms,according to the present invention;

FIG. 8 is a diagram illustrating a further embodiment for coupling lightfrom an OLED into a light-guide optical element utilizing two couplingprisms, according to the present invention;

FIG. 9 is a diagram illustrating a method for coupling light from anLCOS into a light-guide optical element utilizing three coupling prisms,according to the present invention;

FIG. 10 is a diagram illustrating an embodiment for coupling light froman LCD into a light-guide optical element utilizing three couplingprisms and three lenses, according to the present invention;

FIG. 11 is a diagram illustrating an embodiment for coupling light froman LCOS into a light-guide optical element utilizing three couplingprisms and three lenses, according to the present invention;

FIGS. 12 a and 12 b are diagrams illustrating other embodiments forcoupling light into a light-guide optical element utilizing a singlecoupling prism, according to the present invention;

FIG. 13 is a diagram illustrating a view of a device for coupling lightinto a light-guide optical element utilizing a single coupling prism,according to the present invention;

FIG. 14 is a diagram illustrating an embodiment for coupling light outof a light-guide optical element utilizing air gaps, according to thepresent invention;

FIG. 15 is a diagram illustrating another view of a device for couplinglight out of a light-guide optical element utilizing air gaps, accordingto the present invention;

FIG. 16 is a diagram illustrating an embodiment for coupling light froman LCD into a light-guide optical element, utilizing a reflectingsurface which is located next to one of the major surfaces of thelight-guide optical element, according to the present invention, and

FIG. 17 is a diagram illustrating an embodiment for coupling light froman LCOS into a light-guide optical element, utilizing a reflectingsurface which is located next to one of the major surfaces of thelight-guide optical element, according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to substrate-guided optical devices andparticularly to devices which include a plurality of reflecting surfacescarried by a common light-transmissive substrate or to a LOE.

Accordingly, an object of the present invention is to find a coupling-inmechanism different from the coupling-in mechanism of the prior art,replacing the typically used input mirror. There is illustrated in FIG.1 a span of rays that have to be coupled into a LOE, e.g., a substrate20, with a minimal required input aperture 21. In order to avoid animage with gaps or stripes, the points on the boundary line 24, betweenthe edge of input aperture 21 and the lower surface 26 of the substrate20, should be illuminated for each one of the input light waves by twodifferent rays that enter the substrate from two different locations:one ray 30 that illuminates the boundary line 24 directly, and anotherray 31, which is first reflected by the upper surface 32 beforeilluminating the boundary line 24. The size of the input aperture 21 isusually determined by two marginal rays: the rightmost ray 34 of thehighest angle of the FOV, and the leftmost ray 36 of the lowest angle ofthe FOV.

The simplest possibility to couple these rays into the substrate 20 isillustrated in FIG. 2. Here, the input light waves source 38, as well asthe collimating lens 40, are oriented at the required off-axis anglecompared to the major plane of the substrate 20. A relay prism 44 islocated between the collimating lens 40 and the substrate 20 and isoptically cemented to the lower surface 26, such that the light from thedisplay source is trapped inside the substrate by total internalreflection. Although the optical system illustrated here is simple, itstill suffers from a major drawback in that the overall optical systemis large and cumbersome, and does not conform to the external shape ofan eyeglasses display, as well as to a hand-held display.

Therefore, the challenge is to find a method to fabricate a compactcollimating module which can couple light into an LOE having a simpleconfiguration, wherein the overall shape and size of this moduleconforms to most of the relevant applications. A method which achievesthese two seemingly contradictory requirements and which exploits thefact that in most microdisplay sources, such as LCD or LCOS, light islinearly polarized, is illustrated in FIG. 3. The main differencebetween this embodiment and that of FIG. 2 is that the opticalcollimating module is folded, and hence, its volume and weight aresignificantly reduced. Moreover, its mechanical elements can be easilyattached to almost any relevant optical system. As illustrated, thep-polarized input light waves from the input light waves source 38 passthrough a first relay prism 46, a polarizing beamsplitter 48 and asecond relay prism 50. The light waves then pass through thequarter-wavelength retardation plate 52, are collimated by a lens 54,e.g., a plano-convex lens, at its reflecting surface 56, which fromthere are returned again to pass through the retardation plate 52 andre-enter the second relay prism 50. The now s-polarized light waves arereflected from the polarizing beamsplitter 48 and enter the substrate 20through the lower surface 26. The optical module which is illustrated inFIG. 3 is much more compact than that of FIG. 2. For most applications,however, this module is still not compact enough. For example, assume adisplay source having a lateral extent of 9 mm, a horizontal FOV of 22°and prisms having a refractive index of ˜1.51 yields a distance of ˜35mm between the input light waves source 38 and the collimating lens 54.Moreover, for many applications a single reflecting collimating surfacemight not be sufficient to achieve the required optical performance oflow aberrations and distortion. Therefore, an even more compact versionhaving two reflecting collimating surfaces is presented, as illustratedin FIG. 4. The s-polarized input light waves from the input light wavessource 38 pass through a first relay prism 58, a first polarizingbeamsplitter 60 and a second relay prism 62. The waves then pass throughthe quarter-wavelength retardation plate 64, are partially collimated bythe first lens 66 at its reflecting surface 68, returned again to passthrough the retardation plate 64, and re-enter the second relay prism62. The now p-polarized light waves are reflected from the firstpolarizing beamsplitter 60 and following total internal reflection offthe upper surface 70 pass through a second polarizing beamsplitter 72and a third relay prism 74. The waves then pass through thequarter-wavelength retardation plate 76, are collimated by the secondlens 78 at its reflecting surface 80, returned again to pass through theretardation plate 76, and re-enter the third relay prism 74. The nows-polarized light waves are reflected from the second polarizingbeamsplitter 72 and enter the substrate 20 through the lower surface 26.Since the optical way between the input light waves source 38 and thelast collimating surface 80 is now thrice folded, the optical module canbe much more compact than that of FIG. 3. Moreover, better opticalperformance can be achieved by utilizing the two collimating elements 68and 80.

Two different polarizing beamsplitters are utilized in the moduleillustrated above. While the first one 60 transmits an s-polarized lightand reflects a p-polarized light, the second one 70 has the oppositefunction, that is, it transmits a p-polarized light and reflects ans-polarized light. These two different beamsplitters may be materializedutilizing the wire-grid technology which is described in one ofApplicant's previous patent Applications. In the present case, the wiresof the beamsplitters 60 and 70 are oriented parallel and normal to thefigure plane, respectively.

The second relay prism 62 is optically cemented to the left edge of thesubstrate 20, as illustrated in FIG. 4. The reason for doing so is thatthe leftmost rays of the optical waves (such as ray 34) pass from theprism 62 through the surface 26 into the third prism 74. There arecases, however, where it is required to separate between the edge of thesubstrate 20 and the edge of the collimating module, for example, ineyeglasses display applications, wherein the substrate should beassembled inside the eyeglasses frame.

FIG. 5 illustrates a modified version where the upper surface 70 of theprism 62 is aligned with the lower surface 26 of the substrate, ratherthen with the upper surface 28 as illustrated in FIG. 4. In this case,not only the external shape of the module should be modified, but theoptical design of the collimating lenses should be modified as well, inorder to adjust the new optical module to the required focal lengthalong with the desired performances.

The collimating optical modules illustrated in FIGS. 4 and 5 are onlyexamples of different embodiments to reduce the volume of the requiredmodule. Even more compact modules, where a larger number of foldingsurfaces are utilized, are illustrated in FIGS. 6 and 7. As shown, thes-polarized input light waves from the input light waves source 38 passthrough a first relay prism 82, a first polarizing beamsplitter 84 and asecond relay prism 86. The waves then pass through thequarter-wavelength retardation plate 88, are partially collimated by thefirst lens 90 at its reflecting surface 92, returned again to passthrough the retardation plate 88, and re-enter the second relay prism86. The now p-polarized light waves are reflected from the firstpolarizing beamsplitter 84 and following reflection off the reflectingsurface 94 and total internal reflection off the upper surface 96 passthrough a second polarizing beamsplitter 98 and a third relay prism 100.The waves then pass through the quarter-wavelength retardation plate102, are collimated by the second lens 104 at its reflecting surface106, returned again to pass through the retardation plate 102, andre-enter the third relay prism 100. The now s-polarized light waves arereflected from the second polarizing beamsplitter 98 and enter thesubstrate 20 through the lower surface 26. Since the optical pathbetween the input light waves source 38 and the last collimating surface106 is now folded four times, the optical module can be much morecompact then that of FIGS. 4 and 5. As before, the difference betweenthe modules illustrated in FIGS. 6 and 7 is that the upper surface 96 ofthe collimated module in FIG. 6 is aligned with the upper surface 28 ofthe substrate 20 and the right edge of the substrate is opticallycemented to the left edge of the second prism 86, while the uppersurface 96 in FIG. 7 is aligned with the lower surface 26 of thesubstrate 20.

In all the modules illustrated in FIGS. 4 to 7, the input light wavessource 38 is assembled at the rear part of the optical module. Thisarrangement is especially advantageous for systems wherein the displaysource is an LCD and a backlight module should be added to the back sideof the display, however, there are systems wherein the display source isan OLED, which does not require a backlight module, and usually has avery flat shape. In this case, it is possible to place the displaysource at the front part of the optical module, that is, at the samelevel as the LOE, and an even more compact module might be produced.

In FIG. 8, the s-polarized input light waves from the display source 108pass through a first relay prism 110, a first polarizing beamsplitter112 and a second relay prism 114. The waves then pass through thequarter-wavelength retardation plate 116, are partially collimated bythe first lens 118 at its reflecting surface 120, returned again to passthrough the retardation plate 116, and re-enter the second relay prism124. The now p-polarized light waves are reflected from the firstpolarizing beamsplitter 112 and pass through a second polarizingbeamsplitter 122 and a third relay prism 124. The waves then passthrough the quarter-wavelength retardation plate 126, are collimated bythe second lens 128 at its reflecting surface 130, returned again topass through the retardation plate 126, and re-enter the third relayprism 124. The now s-polarized light waves are reflected from the secondpolarizing beamsplitter 122 and enter the substrate 20 through the lowersurface 26. Usually the light from an OLED is unpolarized, in whichcase, in order to avoid scattering from the undesired p-polarized lightfrom the OLED, an s-polarizer 132 must be inserted between the displaysource 108 and the first relay prism 110.

Another advantage of the proposed imaging method illustrated heremanifests itself when utilizing an LCOS device as the display source.Like LCD panels, LCOS panels contain two-dimensional array of cellsfilled with liquid crystals that twist and align in response todifferent voltages. With LCOS, the liquid crystal elements are grafteddirectly onto a reflective silicon chip. According to the liquidcrystals twist following reflection of the mirrored surface below, thepolarization of the light is either changed or unchanged, respectively.This, along with a polarizing beamsplitter, modulates the light andcreates the image. In addition, the reflective technology means theillumination and imaging light beams share the same space. Both of thesefactors necessitate the addition of a special beam-splitting element tothe optical module in order to enable the simultaneous operations of theilluminating, as well as the imaging functions. The addition of such anelement would normally complicate the optical module and when using anLCOS as the display source, the arrangements illustrated in FIG. 2 wouldbecome even larger. For the imaging method illustrated in FIG. 8,however, it is readily possible to add the illuminating unit to theoptical module without significantly increasing the volume of thesystem.

Referring to FIG. 9, the illuminating light waves 134 from the lightsource 136 pass through an s-polarizer 138 and are coupled into thesubstrate 140 by the first reflecting surface 142. Following totalinternal reflection off the upper surface 144 of the substrate, thewaves are reflected and coupled-out off a polarizing beamsplitter 112 toilluminate the LCOS display source 146. Naturally, the number ofelements that could be utilized in the collimating module is not limitedto two.

FIGS. 10 and 11 illustrate a collimating lens, having an LCD and an LCOSas the display sources respectively, wherein a third lens 148 is addedto the optical train. In FIG. 11 another relay prism 150 is added inorder to enable the waves from the light source 136 to illuminate theLCOS 146. In general, for each specific system, the number and types oflenses in the optical collimating module will be set according to therequired optical parameters, desired performance and the volume allowed.

In all the optical collimating modules illustrated in FIGS. 2 to 11, theoff-axis angles of the span of the rays that have to be coupled into thesubstrate, are set by the collimating module. There are cases, howeverwhere it is required to utilize a collimated light waves that impingesthe substrate, normal to the substrate plane. In these cases, analternative coupling-in mechanism should replace the input mirror or theprior art.

Referring to FIG. 7 of Publication WO 2005/024969, another problemrelating to the input mirror 16 of the Publication, which is embeddedinside the substrate 20, is associated with the angular range of theoptical waves which can be coupled inside the LOE by total internalreflection. Similarly to that illustrated in FIG. 1, in order to avoidan image with gaps or stripes, the points on the boundary line betweenthe edge of the input mirror 16 and the upper surface of the substrate20 should be illuminated for each one of the input waves by twodifferent rays that enter the substrate from two different locations. Toenable this requirement, it is necessary to fulfill the followingcondition:α_(sur)>α_(in) ^(max),  (1)wherein:

-   -   α_(in) ^(max) is the maximal off-axis angle of the coupled waves        inside the substrate 20, and    -   α_(sur) is the off-axis angle of the input mirror.

Assuming that the central wave of the source is coupled out of thesubstrate 20 in a direction normal to the substrate surface 26, that theoff-axis angle of the central coupled wave inside the substrate 20 isα_(in), and that the FOV inside that substrate is α_(F), yields:

$\begin{matrix}{\alpha_{sur} > {\alpha_{i\; n} + {\frac{\alpha_{F}}{2}.}}} & (2)\end{matrix}$

The angle α′_(s), between the input mirror and the substrate plane is:

$\begin{matrix}{{\alpha_{sur}^{\prime} = \frac{\alpha_{i\; n}}{2}},} & (3)\end{matrix}$wherein:α′_(sur)=90°−α_(sur).  (4)Combining Eqs. (2)-(4) yields:

$\begin{matrix}{\alpha_{sur} > {{60{^\circ}} + {\frac{\alpha_{F}}{6}.}}} & (5)\end{matrix}$Inserting Eqs. (3)-(4) into Eq. (5) yields:

$\begin{matrix}{\alpha_{i\; n} < {{60{^\circ}} - {\frac{\alpha_{F}}{3}.}}} & (6)\end{matrix}$

For most applications, α_(F)>12°, and therefore, α_(in)<56°. For manyapplications, it is preferred that the off-axis angle of the coupledwaves inside the LOE will be higher, e.g., between 60°-75°, or evenhigher. Another related problem is associated with the maximal possibleFOV. Assuming that the refractive index of the substrate is ˜1.51, theminimal off-axis angle inside the substrate is 42°.

Combining Eqs. (9)-(13) yields:α_(F)<21.6°.  (7)

As a result, the maximal FOV in the air that can be coupled inside theLOE is lower than ˜33°. Hence, utilizing the input mirror of the priorart, which is embedded inside the LOE for coupling-in the incomingwaves, imposes limitations on the FOV of the image, as well as theangular range of the coupled waves inside the LOE.

As illustrated in FIG. 12 a, the lower surface 152 of a coupling-inprism 154 is optically cemented to the substrate 20 at the upper surface28 of the substrate. The collimated light waves from the display source(not shown) pass through the substrate 20 and the prism 154 and are thenreflected from the reflecting surface 156. After again passing throughthe prism 154, the light waves are coupled into the substrate by totalinternal reflection. Similarly to that illustrated above in FIG. 1, inorder to avoid an image with gaps or stripes, the points on the boundaryline 24 between the lower surface 152 of the prism 154 and the uppersurface 28 of the substrate 20 should be illuminated for each one of theinput waves by two different rays that enter the substrate from twodifferent locations: one ray 30 first passes through prism 154, isreflected by the reflecting surface 156, and from there, illuminates theboundary line 24. Another ray 31, is first reflected by the reflectingsurface 156 and then by the lower surface 26 of the substrate 20, beforeilluminating the boundary line. The size of the input aperture isusually determined by two marginal rays: the rightmost ray 34 of thehighest angle of the FOV and the leftmost ray 36 of the lowest angle ofthe FOV. To avoid undesired reflections from the left surface 158, itcan be coated by an opaque obstructive layer. Since the height H of theprism 154 could be larger than the thickness T of the substrate 20, theoff-axis angles of the coupled waves inside the substrate 20 can belarger then the off-axis angle of the reflecting surface 156. As aresult, the images have a much wider FOV with no limitation on themaximal off-axis angles, and therefore, may be coupled into thesubstrate.

FIG. 12 b illustrates a modified version of the coupling-in prism 154wherein the lower surface 152 of the prism is oriented co-linear to thelower surface 26 of the substrate 20. Although the fabrication processof the combined prism-substrate element is more complicated itsadvantage is that the volume of the entire optical system is muchsmaller.

As illustrated in FIG. 13, the off-axis angle of the coupled waves 30,31 inside the substrate 20 is larger than the off-axis angle of thecoupling-in reflecting surface 156, as well as that of the coupling-outpartially reflecting surface 22. In the Publication WO 01/95027, thereis described a system having off-axis angles of the coupled waves whichare larger than the off-axis angles of the coupling-out partiallyreflecting surfaces. The off-axis angle, however, of the coupling-outpartially reflecting surfaces in that case is fairly small, e.g.,approximately 30°, which yields a system having a large number ofpartially reflecting surfaces and hence, an optical element which iscomplicated to fabricate. On the other hand, utilizing the coupling-inmechanism of the present invention, an optical system having an off-axisangle of the coupling-in reflecting source 156, as well as that of thecoupling-out partially reflecting surfaces 22, of between 50° to 60° caneasily be fabricated. In this case, the off-axis angle of the centralcoupled wave 30 inside the substrate 20, will be between 62° to 80°.

So far, it was assumed that the partially reflecting surfaces 22 areeither coated with angular-sensitive coatings or havepolarization-sensitivity characteristics. For images with a relativelynarrow FOV however, it is also possible to utilize a much simplersolution wherein the desired reflections can be achieved with theFresnel reflections from the surfaces. That is, instead of coating thesurfaces, a thin air gap can be inserted between the uncoated surfaces.

FIG. 14 illustrates a substrate, wherein air gaps 160 are insertedbetween the coupling-out surfaces 22. The main drawback of thisconfiguration is the undesired reflectance of the rays having aninternal angle of α_(in). Apparently, the point in the display source,is reflected into the directions α_(sur)−ε and 180−α_(sur)+ε inside thesubstrate. While the ray 162 with the off-axis direction 180−α_(sur)+εis reflected by the partially reflecting surfaces 22 into the requiredoutput direction, the ray 164 with the direction α_(sur)−ε is reflectedby the partially reflecting surfaces 22 into the undesired outputdirection α_(sur)+ε. The reflected ray 166 is then reflected in anundesired direction 2ε, which creates a ghost image. Although only asmall part of the beam is reflected in the undesired direction, theeffect thereof becomes more significant as the FOV is increased; it candisturb the viewer, especially at the extreme of the FOV.

Despite the fact that the undesired reflections described above cannotbe avoided, the problem of ghost images can be solved by changing theangle of the first reflective surface 22. For instance, if this angle ischanged to α_(sur)=63°, the other parameters of the system become:α_(in)=54°; α_(sur)=63°; ε=9°.

Hence, if the FOV of the system is 16° and the refractive index of thesubstrate is 1.51, the maximal off-axis angle of the image inside thesubstrate is 60°, the direction of the undesired reflection is 66°, andthe output angle will be 18°, which is outside the FOV and, with properdesign, will not illuminate the exit pupil. In addition to the ghostimage phenomena, there is another problem; dark stripes are produced bythe undesired reflections of the rays having the direction α_(sur)−ε bythe partially reflecting surfaces 22 into the undesired output direction.alpha.α_(sur)+ε. The dark stripes problem becomes more severe as.epsilon. becomes larger. In addition, since the stripes appear mainlyat the left part of the couple-out aperture of the substrate, theproblem is more severe for the lower angles in the FOV. As a result, forsystems having a central input angle α_(in) which is smaller than thesurface's off axis angle α_(sur)ε is larger for the lower angles of theFOV, therefore the stripes problem is intensified in this region. Withthe optical design presented here, it is possible to fabricate an LOEhaving a central input angle α_(in) which is larger than the surface offaxis angle α_(sur). In that case ε becomes smaller for the lower anglesof the FOV and the stripes problem is significantly decreased.

As illustrated in FIG. 15, the off axis angle α_(in) of the coupledwaves 303 inside the substrate 20 is larger than the off-axis angleα_(sur), of the coupling-out partially reflecting surfaces 22 and thestripes problem is indeed reduced.

In all the optical collimating modules illustrated in FIGS. 2 to 11, theoff-axis angles of the span of the rays that have to be coupled into thesubstrate are set by the collimating module, wherein in the modulesillustrated in FIGS. 12 to 15, the collimated light waves impinge thesubstrate normal to the substrate plane. In the later cases, reflectingelements which are attached to the upper surface of the substrate,produce the coupling-in mechanism which replace the input mirror of theprior art.

FIGS. 16 and 17 illustrate an intermixed solution wherein a collimatingmodule and a reflecting element which are attached to the lower and theupper surfaces of the substrate respectively, are combined to form acompact coupling-in mechanism. As illustrated in FIG. 16, thes-polarized light waves 170 from the light source 172 are coupled intothe substrate 174 by the reflecting surface 176. Following totalinternal reflection off the lower surface 178 of the substrate, thewaves are reflected and coupled-out off a polarizing beam-splitter 180.The waves then pass through the quarter-wavelength retardation plate182, are collimated by the lens 184 at its reflecting surface 186,returned again to pass through the retardation plate 182, and re-entersubstrate 174. The now p-polarized light waves pass through thepolarizing beamsplitter 180 and a relay prism 188. The waves then passthrough the LOE 20 and a second quarter-wavelength retardation plate190, are reflected by the reflecting surface 192, returned again to passthrough the retardation plate 190 and the LOE 20, and re-enter the relayprism 188. The now s-polarized light waves are reflected from thepolarizing beamsplitter 180 and enter the LOE 20 through the lowersurface 26. In FIG. 17 another relay prism 194 is added in order toenable the waves from the light source 196 to illuminate the LCOS 198.The flat reflecting surface 192 in FIGS. 16 and 17 can be replaced by areflecting lens. In that case the collimating of the light waves isperformed by the combined optical power of the reflecting surfaces 186and 192.

In all the optical modules illustrated in FIGS. 1 to 17, the inputaperture of the LOE, namely, substrate 20, is located adjacent to one ofits two major surfaces. The output aperture of the optical means forcoupling light into the substrate by total internal reflection (which islocated outside of said substrate), is optically attached to the inputaperture of the LOE, wherein the optical waves are reflected by areflecting element which is located outside of the LOE and slanted at anangle to the major surfaces of the LOE. In addition, the part of the LOEwhich is located next to the LOE's input aperture is essentiallytransparent and does not contain any reflecting surfaces or any otheroptical elements. Therefore, the fabrication process of such an LOBwould be much simpler than those of the prior art. Moreover, during theassembly process, it is possible to determine the exact orientation ofthe LOE and the exact place of the input aperture.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. An optical device, comprising: alight-transmitting substrate having a first part including an inputaperture surface, a second part including an output aperture surface andat least first and second major surfaces parallel to each other andedges; at least one partially reflecting surface for coupling out lightwaves from the substrate, the partially reflecting surface being locatedin the second part of the substrate and is nonparallel to the majorsurfaces of the substrate; optical means, outside of the substrate, forchanging the direction of incident light waves impinging on the opticalmeans so that the light waves are trapped inside the substrate by totalinternal reflection, and wherein the optical means has an outputaperture surface, the output aperture surface of the optical means beingoptically cemented to the input aperture surface of the substrate, thefirst part of the substrate between the two major surfaces located nextto the substrate input aperture surface, including the input aperturesurface being substantially transparent, the trapped light waves passingthrough the output aperture surface without any substantial reflectionfrom one of the first and second major surfaces of the substrate, theonly optical element inside the substrate being the at least onepartially reflecting surface located in the second part of the substrateand the input aperture surface of the substrate being located at one ofthe two major surfaces of the substrate.
 2. The optical device accordingto claim 1, wherein the part of the substrate which is located next tothe substrate input aperture does not contain an optical element.
 3. Theoptical device according to claim 1, wherein said optical meanscomprises at least one transparent prism having a plurality of surfaces.4. The optical device according to claim 3, further comprising at leastone imaging lens, having at least two surfaces.
 5. The optical deviceaccording to claim 1, further comprising an input light waves source. 6.The optical device according to claim 5, further comprising at least oneretardation plate located in an optical path between said input apertureand said input light waves source.
 7. The optical device according toclaim 6, wherein said retardation plate is a quarter-wavelength plate.8. The optical device according to claim 3, wherein said imaging lensand retardation plate are positioned adjacent to the one of the surfacesof the said prism.
 9. The optical device according to claim 6, whereinsaid retardation plate is positioned between an imaging lens and one ofthe surfaces of a prism constituting said optical means.
 10. The opticaldevice according to claim 4, wherein one surface of said imaging lens iscovered with a reflecting coating.
 11. The optical device according toclaim 4, wherein said imaging lens is a plano-convex lens.
 12. Theoptical device according to claim 7, wherein said imaging lens isoptically attached to said quarter-wave plate.
 13. The optical deviceaccording to claim 4, wherein said imaging lens is a collimating lens.14. The optical device according to claim 3, further comprising at leastone additional transparent prism.
 15. The optical device according toclaim 14, further comprising at least one polarizing beamsplitter. 16.The optical device according to claim 15, wherein said polarizingbeamsplitter is positioned adjacent to the one of the surfaces of thesaid first transparent prism.
 17. The optical device according to claim15, wherein said polarizing beamsplitter is positioned between saidfirst and additional transparent prisms.
 18. The optical deviceaccording to claim 15, wherein said polarizing beamsplitter is awire-grid polarizing beamsplitter.
 19. The optical device according toclaim 1, further comprising at least one display source wherein saiddisplay source produces image light waves which are coupled by theoptical means into said substrate by total internal reflection.
 20. Theoptical device according to claim 19, wherein said light waves arelinearly polarized.
 21. The optical device according to claim 18,wherein said display source produces image light waves which are coupledby the optical means into said substrate by total internal reflection,said light waves being linearly polarized, said input light waves sourcebeing positioned adjacent to a surface of one of said transparentprisms.
 22. The optical device according to claim 19, wherein said lightwaves are coupled by said reflecting surface out of said substrate. 23.The optical device according to claim 18 wherein said display sourceproduces image light waves which are coupled by the optical means intosaid substrate by total internal reflection, said light waves beinglinearly polarized and, further comprising a light source.
 24. Theoptical device according to claim 23, wherein said light source ispositioned adjacent to said input light waves source.
 25. The opticaldevice according to claim 23, wherein the optical means comprises atleast one transparent prism having a plurality of surfaces and saidlight source is positioned adjacent to a surface of one said transparentprisms.
 26. The optical device according to claim 9: further comprisingat least one additional polarizing beamsplitter.
 27. The optical deviceaccording to claim 3, wherein one of the surfaces of said transparentprism is a reflecting surface.
 28. The optical device according to claim3, wherein one of the surfaces of said transparent prism is coated by anopaque obstructive layer.
 29. The optical device according to claim 3,wherein one of the surfaces of said transparent prism is opticallyattached to one of the major surfaces of said substrate.
 30. The opticaldevice according to claim 29, wherein said transparent prism isoptically attached to said substrate at the input aperture of saidsubstrate.
 31. The optical device according to claim 4, wherein saidtransparent prism is optically attached to a first side of the majorsurfaces of said substrate and the said imaging lens is located at thefirst side of said first major surface.
 32. The optical device accordingto claim 4, wherein said transparent prism is optically attached to thefirst side of the major surfaces of said substrate and the said imaginglens is located at the second of the major surfaces.
 33. The opticaldevice according to claim 1, wherein at least one of said partiallyreflecting surfaces is coated with an angular sensitive coating.
 34. Theoptical device according to claim 3, further comprising at least onereflecting element.
 35. The optical device according to claim 34,further comprising at least one retardation plate located in an opticalpath between said substrate and said reflecting element.
 36. The opticaldevice according to claim 35, wherein said retardation plate is aquarter-wavelength plate.
 37. The optical device according to claim 34,wherein said transparent prism is optically attached to the first of themajor surfaces of said substrate and the said reflecting element islocated at the side of the second major surface.
 38. The optical deviceaccording to claim 1, wherein said input aperture is located at one ofthe major surfaces of said substrate.
 39. The optical device accordingto claim 27, wherein said reflecting surface is slanted at an angle tothe major surfaces of said substrate.
 40. The optical device accordingto claim 39, wherein the angle between said reflecting surface and thenormal to the major surfaces of said substrate is between 50° to 60°.41. The optical device according to claim 1, wherein the angle betweensaid partially reflecting surface and the normal to the major surfacesof said substrate is between 50° to 60°.
 42. The optical deviceaccording to claim 1, wherein the off-axis angle of the central coupledwave inside the substrate is between 62° to 80°.
 43. The optical deviceaccording to claim 1, wherein the input aperture of said substrate islocated adjacent to one of its two major surfaces.
 44. An opticaldevice, comprising: a light-transmitting substrate having a first partincluding an input aperture surface, a second part including an outputaperture surface and at least first and second major surfaces parallelto each other and edges; at least one partially reflecting surface forcoupling out light waves from said substrate, which surface is locatedin said second part of the substrate, and is non-parallel to the majorsurfaces of said substrate; optical means for changing the direction ofincident light waves impinging on the substrate such that the lightwaves are trapped inside the substrate by total internal reflection,said optical means having an output aperture, wherein said optical meansis located outside the substrate, the output aperture is opticallycemented to the input aperture surface of the substrate and part of thesubstrate which is located next to the substrate input aperture surfaceis substantially transparent and does not contain any reflectingsurface, and the trapped light waves pass through the output aperturewithout any substantial reflection from the input aperture surface ofthe substrate, and wherein the input aperture surface of the substrateis located at one of the two major surfaces of the substrate.